Method of making asymmetrical layered structures from polyalkylene sulfone resins

ABSTRACT

Asymmetrical layered structures such as asymmetrical films are disclosed. The main body section contains a multitude of pores (voids). A thin surface skin is free of pores. The entire structure is fabricated from a polyalkylene sulfone resin in which the alkylene moiety contains 6 to 18 carbon atoms. Processes for preparing the structures also are disclosed. The structures are semi-permeable to gases and are used as membranes to enrich the oxygen content of oxygen/nitrogen mixtures.

This is a division of application Ser. No. 548,386 filed 11-3-83, nowU.S. Pat No. 4,568,579 issued Feb. 4, 1986.

BACKGROUND OF THE INVENTION

The atmosphere is composed of approximately 78% by volume nitrogen and21% by volume oxygen, with other gases, principally argon, constitutingless than 1% by volume. In most industrial processes requiring oxygen,air is used as the oxygen source. For certain industrial processes andin certain medical treatment procedures, there is a need to enrich theoxygen content of air. Where such needs exist, the air is enriched bybeing mixed with essentially pure oxygen obtained by low temperaturefractionation of liquid air. The preparation of such oxygen-enrichedairs is inherently expensive by reason of the large amount of energyrequired to liquify air. It is apparent that there is a long-term andcontinuing interest in the development of lower cost processes forproviding oxygen/nitrogen gas mixtures having an oxygen content inexcess of the 21 volume % level present in air.

It is known that both nitrogen and oxygen can pass through thin films ofcertain polymeric materials such as polyesters, nylons, polyethylene,diene rubbers such as natural rubber and polybutandiene, siliconerubbers, polyalkylene sulfones, and others. The rate at which a gaspermeates or diffuses through a polymeric membrane is defined by Formula(1). ##EQU1## where: J is the gas flow rate (flux) through the membraneexpressed in

(cm³ at STP)·cm⁻² ·sec⁻¹ ·cm Hg⁻¹.

P is the permeability constant of the membrane expressed in

(cm³ at STP)·cm·cm⁻² ·sec⁻¹ ·cm Hg⁻¹.

ΔP is the differential pressure across the membrane expressed in cm Hg.

A is the cross-sectional area of the membrane expressed in cm².

t is the membrane thickness expressed in cm.

The permeability constant, i.e., P in Formula (1), is a function both ofthe polymeric material from which the membrane is fabricated and theparticular gas permeating the membrane. With all known polymericmembranes, the permeability constant for oxygen is higher than thecorresponding permeability constant for nitrogen. The difference betweenthe permeability constants for oxygen and nitrogen suggests that aoxygen/nitrogen gas mixture enriched in oxygen can be prepared bypassing air through a polymeric membrane. The amount of oxygenenrichment possible is a function of the difference in the twopermeability constants, which can be characterized as an idealseparation factor α which is defined by Formula (2).

    α=P.sub.O.sbsb.2 /P.sub.N.sbsb.2                     (2)

While the possibility of enriching the oxygen content of oxygen/nitrogenmixtures by passing air through a polymeric membrane is theoreticallyattractive, to date a number of factors have prevented any significantuse of this technique. For such a system to be economically attractive,the membrane selected should have both a high permeability constant foroxygen and a large ideal separation factor. Permeability constants andideal separation factors for oxygen and nitrogen obtained with a numberof representative polymers are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Polymer     P.sub.O.sbsb.2.sup.(1)                                                                    P.sub.N.sbsb.2.sup.(2)                                                                αO.sub.2 /N.sub.2                       ______________________________________                                        Polyethylene                                                                              0.022       0.005   4.4                                           Terephthalate                                                                 Nylon 6     0.038       0.010   3.8                                           Butyl Rubber                                                                              1.3         0.31    4.2                                           Polyethylene                                                                              3.0         1.3     2.3                                           Polystyrene 6.4         2.2     2.9                                           Natural Rubber                                                                            30          12      2.7                                           Silicone Rubber                                                                           600         260     2.3                                           ______________________________________                                         .sup.(1) P.sub.O.sbsb.2 × 10.sup.-10                                    .sup.(2) P.sub.N.sbsb.2 × 10.sup.-10                               

The above data indicate that, invariably, polymers having large idealseparation factors have low permeability constants for oxygen.Similarly, polymeric materials having large permeability constants foroxygen have relatively small separation factors. Thus, to obtainsignificant oxygen enrichment of oxygen/nitrogen mixtures, it isnecessary to accept low oxygen permeability constants. To obtainsignificant gas flow across a membrane, it is necessary to accept lowlevels of oxygen enrichment.

To obtain significant flow rates through a membrane, the membranesemployed must be very thin. Obviously, very thin membranes are difficultto fabricate and are quite fragile and subject to frequent breakage.

It is apparent that, if oxygen enrichment of oxygen/nitrogen gasmixtures is to be obtained on a practical basis, it is essential thatthe art develop thin gas permeable membranes which have reasonablemechanical strength, have reasonably high oxygen permeabilitycoefficients, and have reasonably high O₂ /N₂ separation factors. It isthe principal object of this invention to prepare such membranes.

SUMMARY OF THE INVENTION

The invention is directed to gas permeable asymmetrical layeredstructures of polyalkylene sulfone resins having oxygen permeabilityconstants and oxygen/nitrogen separation factors such that theasymmetrical layered structures are well suited for use as a membrane toenrich the oxygen content of an oxygen/nitrogen mixture by passing anoxygen/nitrogen gas mixture through such a membrane The principal bodysection of the asymmetrical structure has a multitude of small poresdistributed substantially uniformly therethrough. One surface of thestructure is homogeneous and is substantially free of pores. The crosssectional thickness of this surface film is substantially thinner thanthe cross-sectional thickness of the principal porous body section. Thelayered structures may take many forms, the principal forms of presentinterest being films and tubes. The entire assymetrical structure isfabricated from a polyalkylene sulfone resin in which the polymerizedalkylene moiety contained therein as an alpha-alkylene which contains 6to 18 carbon atoms.

The invention also is directed to processes for preparing asymmetricallayered structures of polyalkylene sulfone resins of the type describedabove. In the first step of the process, a thin layer of a solution of apolyalkylene sulfone resin is prepared. The solution is saturated with apolyalkylene sulfone resin in a solvent mixture consisting of a goodsolvent for the resin and a poor solvent for the resin. In the next stepof the process, the good solvent for the resin is evaporatedpreferentially from the surface layer of the solution to lower thesolubility of the polyalkylene sulfone resin in the surface layer Thearticle thus formed is next immersed in a water-miscible precipitant toprecipitate essentially all of the polyalkylene sulfone resin containedin the layered structure. Finally, the article is contacted with anon-solvent and/or water for a time period sufficient to extracttherefrom essentially all of the polyalkylkene sulfone solvent and theprecipitant

In further embodiments, the invention is directed to apparatus forenriching the content of a desired gas in a gas mixture. Such apparatusincludes a membrane fabricated from an asymmetrical layered structure asdescribed above. Processes for enriching the oxygen content ofoxygen/nitrogen mixtures by use of such apparatus are also includedwithin the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a section of an asymmetrical film of theinvention.

FIG. 2 is a perspective view, partially in section 3 of an asymmetricaltube of the invention.

FIG. 3 is a side elevation, partially in section, of apparatus forenriching a gas present in a gas mixture employing an asymmetric film ofthe invention as the permeable membrane.

FIG. 4 is a side elevation, partially in section, of apparatus forenriching a gas present in a gas mixture employing as the permeablemembrane tubing of the invention which has an asymmetrical wall.

DETAILED DESCRIPTION OF THE INVENTION

An asymmetrical polyalkylene sulfone film of the invention isillustrated in FIG. 1, which is photomicrograph at a magnification of1500X taken through a section of the film prepared in Example 1,subsequently described. The structure includes a main body section 10having a multiplicity of pores 12 distributed substantially uniformlytherethrough. The pores 12 occupy approximately 50 volume % of the mainbody section of the structure. The surface layer 14 is quite thin and issubstantially free of pores or other openings. Typically, layer 14 willhave a thickness on the order of 1 micron.

FIG. 2 is a drawing of a tube having an asymmetrical wall section. Thewall has a main body section 20 having a plurality of pores 22 uniformlydistributed throughout section 20. The thin outer surface or skin 24 ofthe wall is substantially free of pores or other openings The thicknessof surface layer or skin 24 is substantially the same as the thicknessof the surface layer 14 of the film shown in FIG. 1 and described above.The pores 22 occupy about 50 volume % of the main body section 20. Acenter core or gas passageway 23 is provided in the tube.

The asymmetrical layered structures of the invention are fabricated frompolyalkylene sulfone resins in which the alkylene groups polymerizedtherein are alpha-alkylenes which contain 6 to 18 carbon atoms. Suchpolyalkylene sulfone resins and their method of preparation aredescribed in the prior art. See for example U.S. Pat. No. 3,928,294 andthe references discussed therein, the disclosure of such patent and thereferences referred to therein being incorporated herein by reference.Such resins have a structure in which each polymerized alkylene group islinked to two sulfone groups in a head-to-tail arrangement. Severalproperties of such polyalkylene sulfone resins are described in U.S.Pat. No. 3,928,294, including the oxygen permeability rate through suchresins.

The polyalkylene sulfone resins from which the asymmetrical layeredstructures are fabricated have low to intermediate oxygen permeabilityconstants and good separation factors. When employed as conventionallyprepared films or tubes, they suffer from the prior art deficienciespreviously discussed i.e., they are fragile and lack strength whensufficiently thin to provide a good gas flux, or have a poor gas fluxwhen fabricated as thicker films having better strength.

By reason of their physical structure, the asymmetrical layeredstructures of the invention have a unique combination of good physicalstrength, good gas flux values, and good separation factors. Therelatively thick porous layer provides adequate physical strength to thestructures. By reason of its pores, which are at least partiallyinterconnected, the thick porous layer is very permeable to all gases.The surface layer is very thin and, for this reason, the gas fluxthrough the layered structure is quite good, not withstanding therelatively low gas permeability constants of the polyalkylene sulfoneresin from which the structure is fabricated. In typical articles of theinvention, the porous layer will have a thickness in the range of about0.004-0.02 inch and preferably about 0.008-0.012 inch. The non-poroussurface layer will be as thin as possible, typically in a range of fromabout 0.02 micron to a maximum of about 10 microns. Asymmetrical layeredstructures having such thicknesses typically will have oxygen fluxes(J_(O).sbsb.2) of at least about 1×10⁻⁷ (cm³ at STP)·cm⁻² ·sec⁻¹ ·cmHg⁻¹ as measured across an area of 1 cm² at an pressure of 1 cm Hg.

The asymmetrical films of the invention can be prepared from castingsolutions of critical composition. Such casting solutions consist of anessentially saturated solution of the polyalkylene sulfone resin in amixed solvent system. One component of the casting solution is a goodsolvent having the capability of dissolving at ambient temperature atleast about 30 parts of the polyalkylene sulfone resin in 100 parts byweight of such solvent. The second component of the solventmixture--solely for purposes of differentiating the types of solventsemployed--is referred to as a non-solvent by reason of the fact that itwill have the limited capacity of dissolving not more than 1 part of thepolyalkylene sulfone resin in 100 parts by weight of such non-solvent.

The polyalkylene sulfone resin-containing casting solution will beformulated to contain about 15-30, preferably 20-25, and more especiallyabout 25 weight % of polyalkylene sulfone resin. The solvent/non-solventmixture in which the polyalkylene sulfone resin is dissolved will havethe solvent and non-solvent present in a ratio such that the resultingsolution is essentially saturated with resin at ambient temperature.With the solvent/non-solvent systems preferred for use in the invention,the solvent will be present in a ratio of about 0.5 to 1.5 parts byweight per 1 part by weight of the non-solvent.

The solvent included in the polyalkylene sulfone resin-containingcasting solution must meet several requirements to be useful in thepractice of the invention. Initially, as described supra, the solventmust have the capability at ambient temperature of dissolving at leastabout 30 parts by weight of the polyalkylene sulfone resin per 100 partsby weight of such solvent. In addition, it must have a relatively lowatmospheric boiling point, preferably below about 70° C. In addition, itmust be miscible with the non-solvent, water, and a "precipitant"subsequently described. The preferred solvents for use in theformulation of such casting solutions are aliphatic ketones and ethers.Examples of suitable solvents include acetone and tetrahydrofuran.

The non-solvents included in the casting solutions also must meetseveral criteria. As earlier noted, such non-solvents must have thelimited capacity at ambient temperature of dissolving not more thanabout 1 part by weight of the polyalkylene sulfone resin in 100 parts byweight of such non-solvent. The non-solvent also must have anatmospheric boiling point at least about 5° C. higher than theatmospheric boiling point of the solvent included in the castingsolution. Finally, the non-solvent must be miscible with the solvent,water, and the "precipitant" subsequently described. Alkanols such asmethanol, ethanol, the butanols and the like are the non-solventspresently preferred for use in the processes of the invention.

The casting solutions are prepared most easily and preferably by firstpreparing a concentrated stock solution of the polyalkylene sulfoneresin in the solvent. Typically, such stock solution will contain atleast about 30, and preferably at least about 40, weight % of thepolyalkylene sulfone resin dissolved in the solvent. This stock solutionthen is dilluted with an appropriate quantity of the non-solvent toprepare a casting solution having the desired content of thepolyalkylene sulfone resin.

To prepare asymmetrical films, a thin layer of the casting solution islaid down on a flat surface to prepare a layer having a thickness on theorder of about 0.02 inch. The wet film as cast is permitted to standeither at ambient temperature or at a moderately elevated temperaturefor a period of about 0.05-2.0 minutes. The solvent contained in thecasting solution, by reason of having a lower boiling point than thenon-solvent, evaporates from the surface of the casting solution at afaster rate than the non-solvent. As the concentration of thenon-solvent is increased in the surface layer, the solubility of thepolyalkylene sulfone resin in the surface layer is decreased. Some resinprecipitation may take place, possibly as a gel swollen with solvent.The structure then is immersed into a liquid "precipitant" which ismiscible with both the solvent and the non-solvent originally present inthe casting solution. The precipitant will have a limited ability todissolve the polyalkylene sulfone resin, and frequently will beidentical to the non-solvent included in the original casting solution.As the precipitant is miscible with both the solvent and the non-solventof the casting solution, it penetrates the entire film structure andprecipitates essentially all of the polyalkylene sulfone resin.

In the final step of the process, the bilayered film structure from theprevious step is washed with a non-solvent and/or water to remove all ofthe solvent from the structure. The asymmetrical structure then isdried.

As noted above, the polymer solution cast on a supporting surface ispermitted to stand for a short period of time to permit preferentialevaporation of solvent before the cast solution is contacted with theprecipitant. Although not yet demonstrated experimentally, it may bepossible to omit the evaporation step entirely.

Tubes having asymmetrical wall sections can be prepared in a similarmanner. The polyalkylene sulfone casting solution will be extruded as atube into air, with air or another inert gas being maintained within thecenter of the tube to prevent its collapse. Apparatus and techniques forextruding hollow tubes are disclosed by Cabasso, Klein and Smith,Journal of Applied Polymer Science, Vol. 20, 2377-2397 (1976), whichdescriptions are incorporated herein by reference. The extruded tubethen is fed into a bath containing a precipitant of the type discussedpreviously. The distance between the tip of the extrusion die and theprecipitant bath is set so that the freshly extruded tube is exposed toambient air for about 0.05-2.0 minutes. The solvent contained in thecasting solution, by reason of having a lower boiling point than thenon-solvent, evaporates from the exposed surface of the casting solutionat a faster rate than the non-solvent. As the concentration of thenon-solvent is increased in the surface layer, the solubility of thepolyalkylene sulfone resin in the exposed surface layer is decreased.Some resin precipitation may take place, possibly as a gel swollen withsolvent.

When the extruded tube contacts the liquid precipitant, the precipitantpenetrates the entire film structure and precipitates essentially all ofthe polyalkylene sulfone resin. After being contacted with theprecipitant, the tube then is washed with a non-solvent and/or water toremove all of the solvent from the structure. The asymmetrical structurethen is dried.

FIG. 3 is a side elevation of apparatus which can be employed to enricha gas present in a gas mixture, e.g., oxygen in an oxygen/nitrogenmixture. The apparatus consists of a hollow body 80, which is dividedinto two chambers 80a and 80b by a gas permeable membrane. The membraneconsists of an asymmetrical film of the type shown in FIG. 1 which isattached to a supporting sheet 16 fabricated from a suitable materialsuch as metal to provide strength and rigidity to the membrane. Aplurality of openings 18 are provided in sheet 16 to provide gaspassageways. An inlet 81 is provided for admission of an oxygen/nitrogenmixture into chamber 80a, with an outlet 82 being provided so that theoxygen/nitrogen mixture can be passed continuously through chamber 80a.The apparatus is operated with a pressure differential being created andmaintained across the membrane with the pressure in chamber 80a beinghigher than the pressure in chamber 80b. The pressure differential canbe created by admitting the oxygen/nitrogen mixture through inlet 81 ata pressure somewhat higher than atmospheric pressure, with a one-waycheck valve not shown being provided in outlet 82 to provide a constantpressure in chamber 80a. An outlet 83 is provided in chamber 80b forremoval of the oxygen-enriched oxygen/nitrogen mixture from chamber 80b.In an alternate operating mode, the pump connected to outlet 83 isoperated to maintain a sub-atmospheric pressure in chamber 80b toestablish the pressure differential across the membrane. In thisoperating mode, the pressure in chamber 80a customarily is maintained atatmospheric pressure.

The volume of gas passing across the membrane can be calculated fromformula (1) set forth earlier herein. With a gas mixture such an anoxygen/nitrogen mixture, the total gas passing through the membrane isthe sum of the flux for oxygen (J_(O).sbsb.2) and the flux for nitrogen(J_(N).sbsb.2), i.e.:

    J.sub.total =J.sub.O.sbsb.2 +J.sub.N.sbsb.2                (3)

The flux J for each gas is a function of (a) the permeability constantfor the gas through the membrane, (b) the cross-sectional area of themembrane, (c) the thickness of the membrane, and (d) the pressuredifferential across the membrane, i.e., the pressure in chamber 80aminus the pressure in chamber 80b. Since the gas is present in a mixturewith a second gas, the flux also is a function of its volume fraction inthe gas mixture. Thus, the flux for oxygen in an oxygen/nitrogen mixtureis defined by formula (4): ##EQU2## Similarly, the flux for nitrogen inan oxygen/nitrogen mixture is defined by formula (5): ##EQU3##

The enrichment of the desired gas can be calculated from (1) the volumefraction of the desired gas in the gas mixture, and (2) the idealseparation factor α for the membrane. The separation factors forasymmetrical films prepared from a series of polyalkylene sulfone resinsare set forth subsequently.

Where oxygen-nitrogen mixtures having still higher concentrations ofoxygen are desired, two or more units of the type illustrated in FIG. 3can be connected in a series or cascade arrangement in which theoxygen-enriched oxygen/nitrogen mixture from a first unit is passedsuccessively through a second or a series of such units. With eachpassage of the oxygen/nitrogen mixture through successive units, therewill be a further oxygen enrichment of the oxygen/nitrogen mixture.

FIG. 4 is a side elevation of another form of apparatus for enriching agas present in a gas mixture, e.g., oxygen in an oxygen/nitrogenmixture. The apparatus consists of a cylindrical chamber 90 whichincludes an interior chamber 90a. Tubing of the type shown in FIG. 2 andhaving an asymmetrical wall is employed as the membrane. As illustrated,the apparatus is shown as having a single tubular membrane. It will berecognized, of course, that the apparatus will contain many such tubularmembranes running through the chamber 90a. The membrane includes acentral elongated gas passageway 23, a main wall body 20 having aplurality of pores 22 therein, and a thin outer surface 24. The tubularmember of the invention is encased in another tubular member 26fabricated from material such as metal to provide rigidity and strength.Tube 26 includes a plurality of gas passageways 27.

In operation, an oxygen/nitrogen gas mixture under an elevated pressureis passed through central passageway 23. Both oxygen and nitrogen passthrough the main body section 20 and the non-porous surface layer 24,and finally through openings 27 provided in tube 26. The oxygen-enrichedoxygen/nitrogen mixture from chamber 90a exits the apparatus throughline 91.

By minor structural modifications, the gas flow through the apparatus ofFIG. 4 can be reversed. In such an embodiment, one end of the tubularmembrane will be sealed. The initial oxygen/nitrogen gas mixture thenwill be introduced through line 91. The oxygen-enriched mixture will bewithdrawn through gas passageway 23. The calculations of the total gasflux and the oxygen enrichment will be the same as discussed supra withrespect to FIG. 3. Where additional oxygen enrichment is desired, two ormore units as illustrated in FIG. 4 can be connected in a series orcascade arrangement.

The following examples are set forth to illustrate more clearly theprinciple and practice of the invention to those skilled in the art.Where parts or percentages are mentioned, they are parts and percentagesby weight unless otherwise indicated.

EXAMPLE 1 Part A

A three-liter flask was mounted in a cooling bath filled with crushedice and water. The flask was flushed free of oxygen and moisture bypassing dry nitrogen through the flask for ten minutes. The flask thenwas charged with 1000 ml of ethanol and purged with dry nitrogen for anadditional two-minute period. Liquified SO₂ in the amount of about 300grams was charged to the flask over a period of about one hour. A chargeof about 100 ml of hexene-1 then was charged to the flask with stirring.A dropping funnel was charged with 100 ml of hexene-1 having 1.6 ml oft-butyl hydroperoxide slurried therein. This solution was added dropwiseover a twenty-minute period while the contents of the flask were beingstirred. Polyhexene-1 sulfone began forming immediately as evidenced bythe formation of a milky appearing emulsion. Stirring was continued forapproximately ten hours. The reaction mixture then was allowed to remainin the flask over a sixteen-hour period with nitrogen being passedthrough the flask and with the reaction mixture being allowed to warm toroom temperature. At the end of this period, the contents of the flaskhad a white polymer precipitate on the bottom and a clear supernatentliquid on the top.

The supernatent liquid was drawn off with an aspirator. The polymer wascharged with fresh ethanol to a Waring blender. After vigorous mixingfor a short period, the blender was turned off to permit the polymer tosettle, and the ethanol was drawn off. The polymer then was mixed withan additional aliquot of fresh ethanol and permitted to stand for 24hours. The polymer then was filtered and air-dried in a forced air ovenat 50° C. over a period of five hours.

Part B

A polyalkylene sulfone resin was prepared from octene-1 employing theprocedure described in Part A, above.

Part C

A polyalkylene sulfone resin was prepared from decene-1 employing theprocedure described in Part A, above.

Part D

A polyalkylene sulfone resin was prepared from hexadecene-1 employingthe procedure described in Part A, above.

EXAMPLE 2

An asymmetrical film of polyhexene sulfone was prepared from thepolyhexene sulfone resin prepared in Example 1, Part A. Twenty-fiveparts of the resin were dissolved in 75 parts of a mixed solventconsisting of 60 weight % acetone and 40 weight % methanol. This resinsolution was poured onto a glass plate and drawn down to form a thinfilm approximately 0.020" thick. After standing for approximately oneminute, the glass plate was placed in a stirred bath containingmethanol. After a one-hour period, the fully precipitated film wasremoved from the glass plate and thoroughly washed with water to removeany residual solvent. The asymmetrical film had a porous body containinga thin surface skin which was pore-free. A photomicrograph of across-section of the asymmetrical film at a magnification of 1500X isshown in FIG. 1.

EXAMPLE 3

A polymer casting solution was prepared by dissolving 21 parts of thepolyoctene sulfone resin of Example 1, Part B in 79 parts of a solventmixture containing 47 weight % tetrahydrofuran and 53 weight % ethanol.An asymmetrical film was prepared from this casting solution employingthe same procedure set forth in Example 2. Ethanol was used as theprecipitant in lieu of the methanol employed in Example 2.

EXAMPLE 4

A casting solution was prepared by dissolving 21 parts of the polydecenesulfone resin prepared in Example 1, Part C in 79 parts of a mixedsolvent containing 55 weight % tetrahydrofuran and 45 weight % ethanol.An asymmetrical film was prepared from this casting solution employingthe techniques described in Example 2, except that ethanol was used asthe precipitant.

EXAMPLE 5

A casting solution was prepared by dissolving 25 parts of thepolyhexadecene sulfone resin prepared in Example 1, Part D in 75 partsof a solvent mixture containing 35 weight % tetrahydrofuran and 65%weight % t-butanol. An asymmetrical film was prepared from this castingsolution employing the techniques described in Example 2.

The flux of pure oxygen and the flux of pure nitrogen across theasymmetrical films prepared in Examples 2-5, inclusive, were measuredemploying a modification of the method described by Kammermeyer inModern Plastics, July 1962, at page 135. In this method, a membrane ofgiven cross-sectional area is placed in a filter holder. The gas whoseflux is to be measured is introduced into a chamber on one side of themembrane. The other side of the membrane is a glass chamber of knownvolume fitted with a stopcock. A vacuum is drawn on the system and thepressure on the downstream side of the membrane is measured with amercury manometer. The stopcock then is closed to isolate the systemfrom the vacuum pump, and the change in downstream pressure as afunction of time is noted. From the observed pressure change, the volumeof gas flowing through the membrane can be calculated.

Both the oxygen flux and the nitrogen flux across each of theasymmetrical films prepared in Examples 2-5 were measured and are shownin Table II. For comparison purposes, the corresponding oxygen andnitrogen flux values through non-porous films 0.01 inch thick preparedfrom the polyalkylene resins of Example 1, Parts A-D are shown in TableII. The flux values for the non-porous films are values calculated byFormula (1), with the permeability constant P having been determinedexperimentally on films approximately 0.01 inch thick. Calculated valuesat a uniform film thickness were used so as to provide a uniform basisof comparison for each of the polyalkylene sulfone resins.

                  TABLE II                                                        ______________________________________                                        Non-Porous Film Data                                                                              Asymmetrical Film Data                                                           Sepa-              Sepa-                                      O.sub.2 N.sub.2 ration O.sub.2                                                                             N.sub.2                                                                             ration                              Polymer                                                                              Flux.sup.(1)                                                                          Flux.sup.(1)                                                                          Factor Flux.sup.(1)                                                                        Flux.sup.(1)                                                                        Factor α                      ______________________________________                                        PAS-6.sup.(2)                                                                        0.02    0.005   4.0     1.02  0.26 3.9                                 PAS-8.sup.(3)                                                                        0.1     0.033   3.0     9.8  3.5   2.8                                 PAS-10.sup.(4)                                                                       0.19    0.074   2.5    11.7  4.5   2.6                                 PAS-16.sup.(5)                                                                       0.28    0.12    2.4    10.5  4.2   2.5                                 ______________________________________                                         .sup.(1) × 10.sup.-6                                                    .sup.(2) Resin prepared in Example 1, Part A Asymmetrical Film prepared i     Example 2                                                                     .sup.(3) Resin prepared in Example 1, Part B Asymmetrical Film prepared i     Example 3                                                                     .sup.(4) Resin prepared in Example 1, Part C Asymmetrical Film prepared i     Example 4                                                                     .sup.(5) Resin prepared in Example 1, Part D Asymmetrical Film prepared i     Example 5                                                                

From the data presented in Table II, it will be noted that theseparation factor for each of the polyalkylene sulfone resins isidentical (within experimental error) when employed either as a nonporous film or as an asymmetrical film. This is the expectedrelationship as the premeability constant, i.e., P in formula (1), is afundamental property of the resin itself. If the measured separationfactor for the asymmetrical film is lower than the value shown in TableII, this is evidence that the skin of the asymmetrical film has pinholestherein.

The fluxes for both oxygen and nitrogen are materially higher throughthe asymmetrical films. As a specific example, the oxygen flux in theasymmetrical film prepared from the polyhexene sulfone was 1.02×10⁻⁶,while the oxygen flux through the non-porous film was 0.02×10⁻⁶. Thisvery large difference in flux is accounted for by the fact that thepore-free skin of the asymmetrical film was materially thinner than thefilm employed as the control. The asymmetrical film had a physicalstrength substantially equivalent to that of a solid film 0.01 inchthick prepared from the same resin. These factors clearly indicate thatthe asymmetrical film will be much more efficient than a conventionallyprepared film for use as a membrane in enriching the oxygen content ofan oxygen/nitrogen gas mixture by passing such a mixture through amembrane.

While the articles, processes, and apparatus herein described constitutepreferred embodiments of the invention, it is to be understood that theinvention is not limited to these precise articles, processes, andapparatus, and that changes may be made therein without departing fromthe scope of the invention which is defined in the appended claims.

What is claimed is:
 1. A method for preparing an asymemtrical layeredstructure of a polyalkylene sulfone resin which consists essentially ofthe sequential steps:(a) forming a thin layer of a solution ofpolyalkylene sulfone resin; (b) evaporating solvent from the surfacelayer of the solution of (a) to lower the solubility of the polyalkylenesulfone resin in said surface layer; (c) contacting the article of (b)with a water-miscible precipitant to precipitate essentially all of thepolyalkylene sulfone contained in the layered structure formed in (b) toform an article; and (d) contacting the article formed in (c) with anon-solvent and/or water for a time period sufficient to extracttherefrom essentially all of the polyalkylene sulfone solvent;thepolyalkylene sulfone, the solution thereof and the solvents thereforhaving the following limitations: (e) the polymerized alkylene in saidpolyalkylene sulfone resin is an alpha alkylene which contains 6 to 18carbon atoms; (f) the solution of (a) is essentially saturated withpolyalkylene sulfone and contains therein:(i) A water-miscible organicsolvent which will dissolve therein at ambient temperature at leastabout 30 parts by weight of said polyalkylene sulfone resin in 100 partsby weight of said solvent; and (ii) A non-solvent miscible with bothwater and the solvent of (f) (i), said non-solvent having the capacityof dissolving at ambient temperature not more than about 1 part byweight of said polyalkylene sulfone resin in 100 parts by weight of saidnon-solvent, said non-solvent having an atmospheric pressure boilingpoint at least about 5° C. higher than the atmospheric pressure boilingpoint of the solvent of (f) (i), and (g) the water-miscible precipitantemployed in (c) also is miscible with both the solvent of (f) (i) andthe non-solvent of (f) (ii) and has the capacity of dissolving atambient temperature not more than about 1 part of weight of saidpolyalkylene sulfone resin in 100 parts by weight of said precipitant.2. A process of claim 1 in which the solvent component of thepolyalkylene sulfone resin solution has an atmospheric pressure boilingpoint not higher than about 70° C.
 3. A process of claim 1 in which thepolyalkylene sulfone resin solution includes as the solvent an aliphaticketone or an ether, and as the non-solvent an alkanol.
 4. A process ofclaim 3 in which the solvent is acetone.
 5. A process of claim 3 inwhich the solvent is tetrahydrofuran.
 6. A process of claim 1 in whichthe polyalkylene sulfone resin solution employed in step (a) containsabout 20-25 weight % of said polyalkylene sulfone resin.
 7. A process ofclaim 3 in which the polyalkylene sulfone resin solution employed instep (a) contains about 20-25 weight % of said polyalkylene sulfoneresin.
 8. A process of claim 4 in which the polyalkylene sulfone resinsolution employed in step (a) contains about 20-25 weight % of saidpolyalkylene sulfone resin.
 9. A process of claim 5 in which thepolyalkylene sulfone resin solution employed in step (a) contains about20-25 weight % of said polyalkylene sulfone resin.
 10. A method forpreparing an asymmetrical layered structure of a polyalkylene sulfoneresin which consists essentially of the sequential steps:(a) forming athin layer of a saturated solution of polyalkylene sulfone resin inwhich the alkylene group contains 6 to 18 carbon atoms, the solutioncontaining a water-miscible organic solvent which will dissolve thereinat ambient temperature at least about 30 parts by weight of saidpolyalkylene sulfone resin in 100 parts by weight of said solvent; and anon-solvent miscible both water and the solvent of water-misciblesolvent, said non-solvent having the capacity of dissolving at ambienttemperature not more than about 1 part by weight of said polyalkylenesulfone resin in 100 parts by weight of said non-solvent, saidnon-solvent having an atmospheric pressure boiling point at least about5° C. higher than the atmospheric pressure boiling point of thewater-miscible solvent; (b) evaporating solvent from the surface layerof the solution of (a) to lower the solubility of the polyalkylenesulfone resin in said surface layer; (c) contacting the layer of (b)with a water-miscible precipitant to precipitate essentially all of thepolyalkylene sulfone contained in the layered structure formed in (b) toform an article, the water-miscible precipitant employed in (c) also ismiscible with both the water-miscible solvent and the non-solvent andhas the capacity of dissolving at ambient temperature not more thanabout 1 part by weight of said polyalkylene sulfone resin in 100 partsby weight of said precipitant; and (d) contacting the article formed in(c) with a non-solvent and/or water for a time period sufficient toextract therefrom essentially all of the polyalkylene sulfone solvent toprovide the asymmetrical structure which has an oxygen permeation fluxof at least about 1×10⁻⁷ (cm³ at STP)·cm⁻¹ ·sec⁻¹ ·cm Hg⁻¹ as measuredacross an area of 1 cm² at a pressure differential of 1 cm Hg.